For example, in a plasma processing apparatus described in Patent Document 1, an electrode connected to a power source and an electrode grounded to the earth are arranged in a vertically opposing relation. Each electrode is received in a metal-made frame through a holder composed of an insulating material such as resin. An atmospheric pressure plasma discharge space is formed between those electrodes and a substrate is arranged in this discharge space so as to be subjected to plasma processing. As a cooling medium for the electrode, water is used in general. The electrodes are firmly fixed to the holder of the apparatus main body without play. Patent Document 2 discloses that electrodes are gently held by a holder.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-228136    [Patent Document 2] Japanese Patent Application Laid-Open No. H09-92493